In electronic equipment, electronic parts are disposed atop a circuit of printed circuit board or the like to perform prescribed functions. From long in the past, connection of electronic parts to printed circuit boards or other circuits has been performed using solder, which can perform connection at a low temperature, is inexpensive, and has high bonding reliability. Electronic parts used in electronic equipment need to have terminals such as part leads which are connected to printed circuit boards. Solder is also used inside electronic parts to connect terminals such as part leads to part components which perform the basic functions of the electronic parts.
Solder alloys of Sn and Pb particularly having a composition in the vicinity of 60% Sn which have a low melting point have been used as solder alloys in soldering for electronic equipment. In particular, a 63Sn-37Pb composition has a solidus temperature and a liquidus temperature which are both 183° C., thereby minimizing the formation of cracks at the time of cooling of solder. In addition, as it has the lowest melting temperature among solder alloys of Sn and Pb, it also minimizes damage to electronic parts due to heat. It is so widely used that the term “solder” generally refers to a 63Sn-37Pb solder alloy.
However, if a 63Sn-37Pb solder alloy is used to perform internal bonding of an electronic part, due to the heat which is applied during manufacture of electronic equipment, solder inside an electronic part may melt, thereby causing short circuits or disconnection of part components and leads which are connected by solder, and the electronic part can no longer function properly. Therefore, solders with a higher melting point than a 63Sn-37Pb solder alloy is used for bonding inside electronic parts. These solder alloys are referred to as high-temperature solders in view of their melting points which are high compared to solders such as 63Sn-37Pb used in soldering printed circuit boards
High-temperature solders for use in internal bonding of electronic parts primarily have Pb as a main component and include Pb-10Sn (solidus temperature of 268° C. and liquidus temperature of 302° C.), Pb-5Sn (solidus temperature of 307° C. and liquidus temperature of 313° C.), Pb-2Ag-8Sn (solidus temperature of 275° C. and liquidus temperature of 346° C.), Pb-5Ag (solidus temperature of 304° C. and liquidus temperature of 365° C.), and the like. Since these high-temperature solders have a solidus temperature of at least 260° C., even if the soldering temperature of a 63Sn-37Pb eutectic solder used for soldering of printed circuit boards is a somewhat high value of 230° C., the soldered portions inside electronic parts soldered with a high-temperature solder such as Pb-10Sn do not melt at the time of soldering of printed circuit boards.
Because portions to be connected inside electronic parts are minute, soldering for the purpose of bonding inside electronic parts is carried out by reflow soldering, which is capable of performing fine soldering. Methods of applying solder paste to portions to be soldered include printing, discharging of solder paste using a dispenser, transfer of solder paste by transfer pins, and the like. Solder paste used in reflow soldering is formed by mixing solder powder and a soldering flux. As electronic parts become smaller, the solder powder used in solder paste is becoming finer. Because the surface of such fine solder powder is susceptible to oxidation, solder powder having a clean powder surface is desired.
When soldered electronic equipment breaks or becomes old and difficult to use, it may be discarded instead of being repaired or being used with patience. When electronic equipment is discarded, plastic, glass, metal, and the like which constitute the electronic equipment is sometimes recovered and reused, but since printed circuit boards have copper foil bonded to resin portions and having solder deposited thereon, it is difficult to separate these materials for recovery and reuse, and the circuit boards have been buried after pulverization or as is.
Due to extensive use of fossil fuels in recent years, rain which falls onto the ground is becoming acidic. If acid rain permeates into the ground and contacts printed circuit boards which have been disposed of by burial, the Pb in solder is dissolved out, and acid rain containing Pb further permeates into the ground and is incorporated into underground water. If underground water containing the Pb is drunk by humans for long periods of time, the Pb accumulates in the body and it is said to eventually cause lead poisoning. Therefore, the use of Pb has come to be regulated on a global scale. Naturally, Pb—Sn high-temperature solders and Pb—Sn eutectic solder which have conventionally been used for soldering are also becoming objects of regulation.
In light of the fact that the use of Pb—Sn solder is becoming regulated, the use of lead-free solder which does not contain any Pb is now recommended. Lead-free solder is formed by a combination of two or more elements such as Sn, Ag, Sb, Cu, Zn, Bi, In, Ni, Cr, Fe, P, Ge, and Ga. Examples of binary lead-free solders are Sn-3.5Ag (eutectic temperature of 221° C.), Sn-55b (eutectic temperature of 240° C.), Sn-0.75Cu (eutectic temperature of 227° C.), Sn-58Bi (eutectic temperature of 139° C.), and Sn-52In (eutectic temperature of 117° C.). There are also ternary or higher order systems which combine the above elements to improve the melting point or mechanical properties. At present, lead-free solders which are frequently used include Sn-3Ag-0.5Cu (solidus temperature of 217° C. and liquidus temperature of 220° C.), Sn-8Zn-3Bi (solidus temperature of 190° C. and liquidus of 197° C.), Sn-2.5Ag-0.5Cu-1Bi (solidus temperature of 214° C. and liquidus temperature of 221° C.), and the like. These lead-free solders have a melting temperature which is approximately 40° C. higher than that of a conventional 63Sn-37Pb solder alloy.
However, even if one desired to use a high-temperature lead-free solder for initial soldering due to regulations on the use of Pb, there was no high-temperature solder having Sn as a primary component and having a solidus temperature of at least 260° C. For example, in the case of an Sn—Ag based solder having a solidus temperature (eutectic temperature) of 221° C. or an Sn—Sb based solder having a solidus temperature of 227° C., if the Ag content is increased, the liquidus temperature increases but the solidus temperature does not. In the case of an Sn—Sb based solder, an extreme increase in the content of Sb causes the liquidus temperature to also greatly increase. Addition of other elements to the alloy also causes the solidus temperature to increase, and this behavior cannot be changed. Therefore, it has been thought impossible to use a lead-free solder as a high-temperature solder for internal bonding of electronic parts.
High-temperature solders in which Sn and In are added to Zn, Al, and Ge, and high-temperature solders in which Sn and In are added to Zn, Al, Ge, and Mg have been disclosed as high-temperature solders for internal bonding of electronic parts.
Because these high-temperature solders contain elements such as Zn, Al, and Mg which have poor wettability, the solders are not suitable for use with electronic parts. These conventional high-temperature solders melt to form a single-phase structure. In contrast, JP 2002-254194 A discloses a high-temperature solder which performs bonding by forming an intermetallic compound without forming a single-phase structure by melting.    Patent Document 1: JP 2002-254194 A